Traditionally chain links for making rope chains are produced as follows. A precious metal wire, generally having a circular cross-section, is wound around a manedrel to form a spiral coil. The coil is then cut so that each loop is cut slightly askew of its adjacent loop, resulting in accentric wire links having gaps accentrically aligned. In order to straighten these accentric links to be planar, the links are passed between pressure rollers or other pressure means to force the gap ends into the same plane. An alternative way of making gapped links is by taking a wire of any shape cross-section, cutting it to the desired length of the outer circumference of a link and bending the wire around a former into the desired link shape by pressing the ends of the wire towards the body of the former.
Rope chains have been prepared for decades primarily by hand, using such gapped links. Even today this is by far the dominant method for preparing rope chains. A complete discussion of the manual process for making rope chains can be found in the patent to Benhamou et al, U.S. Pat. No. 4,651,517 and my own U.S. Pat. Nos. 4,934,135 and 4,996,835.
Machines for producing rope chains are also known as disclosed in U.S. Pat. Nos. 4,493,183 and 4,503,664. The machine process also uses wire to prepare the chain links. French Patent 2154 dated June 1846 discloses what appear to be punched jewelry articles in the form of gapped rings. The rings, however, are relatively large, with substantially wide gaps, much larger than the links used for making rope chains. This is quite evident from the scale at the bottom of the drawing. This scale represents a distance of 100 mm (0 to 1 decimeters) with each increment representing 10 mm. Thus the open ring-like articles in this patent all have diameters of about 20 mm and gaps of about 10 mm, as can be seen from the figures. Therefore, the patent relates only to relatively large individual jewelry articles with large open sections (gaps) which can readily made by a single punch method. The patent, however, does not suggest nor teach to use the single punch method for manufacturing rope chain links having very small diameters in the order of 2-5 mm with gaps as narrow as 0.2-0.7 mm. Recently a number of design patents were published, for example the International Design No. DM/014,648 to S.I.L.O. s.P.a. and U.S. Pat. Nos. Des. 340,422 and 343,136 which disclose rope chains and links for preparing these rope chains, having straight sided polygonal shapes such as squares, hexagons and octagons. Such shaped links are difficult to prepare from wire material, even from wires having polygonal cross-sections, because in bending the wire into the shape of a link, there are always rounded corners which require a further step to remove or straighten them.
Thus, using square or rectangular cross-section wire to prepare chain links generally tends to deform the wire cross-section at the bends in the course of bending it into the shape of a chain link. A recent patent to Grando, U.S. 5,309,704, claims a multi-punch process for making chain links with planar surfaces and sharp corners by using progressive punch and die sets to punch out chain links from a sheet of metal foil in a two punch process. This method involves piercing a sheet of metal of appropriate thickness with a first punch and die set, to create the interior of the link, and subsequently, in a second step, piercing the sheet with a blanking punch and die set to a predetermined shape to create the exterior of the link and separating the finished link from the sheet.
In this manner, the link which is produced has planar surfaces and planar sides orthogonal to said surfaces. This double punch process enabled making small links of almost any shape by providing suitable first and second punches of the desired shape. Essentially this process is an adaptation of the generally well-known process of double punching metal sheets or strips to make links for rope chains. It is believed that the double punching method was selected because a single punch method was thought to be non-viable on a commercial scale, because of the thin punch die required for making the gap.